Thief hatches with diaphragm assisted sealing

ABSTRACT

Thief hatches with diaphragm assisted sealing are disclosed. An example apparatus includes a base attachable to a tank, and a cover attachable to the base via a hinge. The example apparatus further includes a vacuum seal assembly couplable to the cover, and a diaphragm coupled to a stem of the vacuum seal assembly. The diaphragm is moveable in response to a pressure differential between a first pressure within the tank and a second pressure exterior to the tank.

FIELD OF THE DISCLOSURE

This disclosure relates generally to thief hatches and, moreparticularly, to thief hatches with diaphragm assisted sealing.

BACKGROUND

Thief hatches are hatches mounted to tanks or other fluid containingvessels that often serve the dual purpose of (1) providing access to thecontents of such tanks or vessels to thieve or extract samples, measurelevels, and/or take other measurements of the contents of the tanks orvessels, and (2) regulating the pressure within such tanks or vessels.Such hatches provide access to the contents of a tank by a cover or lidthat may be pivoted about a hinge to an open position, therebyuncovering an aperture in the tank. Some thief hatches regulate pressurein a tank by releasing pressure when the tank pressure exceeds a highpressure threshold and introducing pressure into the tank when the tankpressure drops below a vacuum pressure threshold (e.g., a negativepressure threshold relative to the ambient atmosphere). When the tankpressure is between the high pressure threshold and the vacuum pressurethreshold, seals in the closed hatch reduce (e.g., prevent) leakage offluid out of the tank. While suitable to any application, such pressureregulating hatches are often implemented on tanks in which volatileand/or vaporizable liquids are stored, transported, or otherwisehandled.

SUMMARY

Thief hatches with diaphragm assisted sealing are disclosed. An exampleapparatus includes a base attachable to a tank, and a cover attachableto the base via a hinge. The example apparatus further includes a vacuumseal assembly couplable to the cover, and a diaphragm coupled to a stemof the vacuum seal assembly. The diaphragm is moveable in response to apressure differential between a first pressure within the tank and asecond pressure exterior to the tank.

Another example apparatus includes a base attachable to a tank, and acover pivotably coupled to the base via a hinge. The example apparatusfurther includes an actuator housing coupled to the cover. The exampleapparatus also includes a diaphragm disposed between the actuatorhousing and the cover. The diaphragm separates a first chamber withinthe actuator housing from a second chamber within the cover. A pressuredifferential between a first pressure in the first chamber and a secondpressure in the second chamber is to produce a force urging a vacuumseal against a sealing surface.

Another example apparatus includes a base of a thief hatch attachable toa tank and a cover of the thief hatch attached to the base via a hinge.The example apparatus further includes a vacuum seal disposed within thethief hatch. The vacuum seal is to be urged against a sealing surfacewith a sealing force that includes a pressure force generated from afirst pressure in the tank when the first pressure is above a secondpressure in an ambient environment exterior to the tank. The vacuum sealis to let pressure into the tank from the ambient environment when atank pressure is less than a vacuum pressure threshold. the exampleapparatus also includes means for providing a larger surface area onwhich the first pressure is applied to increase the sealing force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a storage tank system having an example thiefhatch constructed in accordance with the teachings disclosed herein.

FIG. 2 is a cross-sectional view of the example thief hatch of FIG. 1.

FIG. 3 is a cross-sectional view of another example thief hatch.

The figures are not to scale. Instead, to clarify multiple layers andregions, the thickness of the layers may be enlarged in the drawings.Wherever possible, the same reference numbers will be used throughoutthe drawing(s) and accompanying written description to refer to the sameor like parts. As used in this patent, stating that any part (e.g., alayer, film, area, or plate) is in any way positioned on (e.g.,positioned on, located on, disposed on, or formed on, etc.) anotherpart, means that the referenced part is either in contact with the otherpart, or that the referenced part is above the other part with one ormore intermediate part(s) located therebetween. Stating that any part isin contact with another part means that there is no intermediate partbetween the two parts.

DETAILED DESCRIPTION

Many thief hatches include a vacuum seal assembly and a pressure sealassembly that work in combination to seal off the aperture of a tankwhere the hatch is located when the tank pressure is within anacceptable range relative to an ambient environment (e.g., atmosphericpressure). If pressure within the tank exceeds an upper threshold abovean ambient (e.g., atmospheric) pressure, the pressure seal assemblywithin the thief hatch is designed to break its seal to release somepressure within the tank. If pressure within the tank drops below alower threshold below an ambient (e.g., atmospheric) pressure, therebyforming a vacuum (e.g., a negative pressure), the vacuum seal assemblywithin the thief hatch is designed to break its seal to introducepressure into the tank. When the pressure within the tank is between theupper and lower thresholds, the hatch is designed to maintain the sealsof both the pressure and vacuum seal assemblies in sealing engagementwith their respective sealing surfaces to substantially prevent anyfluid from escaping the tank (e.g., substantially prevent leakage ofvapor, fumes, and/or fluid from passing the seals).

The ability of a thief hatch to prevent leakage passing the seals withinthe hatch depends upon the sealing force applied to the seals. Thepressure seal assembly of many known hatches includes a first seal(referred to herein as a pressure seal) that is urged against a firstsealing surface based on a spring force generated by a first spring(referred to herein as a pressure spring). Similarly, the vacuum sealassembly of many known hatches includes a second seal (referred toherein as a vacuum seal) that is urged against a second sealing surfacebased on the spring force generated by a second spring (referred toherein as a vacuum spring).

The pressure and vacuum springs are designed with a strength or ratingto counteract opposing forces on the respective pressure and vacuumseals produced by the pressure within the tank. More particularly, avacuum (e.g., a negative pressure) in the tank will contribute to,rather than oppose, the sealing force of the pressure seal, while anover pressure will contribute to, rather than oppose, the sealing forceof the vacuum seal. As such, the pressure spring is designed tocounteract positive pressures up to the upper threshold before givingway to release the over pressure. Further, the vacuum spring is designedto counteract negative pressures down to the lower threshold beforegiving way to allow pressure into the tank to reduce the vacuum. Thus,the pressure seal assembly operates in one direction to maintain a tightseal until the tank pressure exceeds the upper threshold, and the vacuumseal assembly operates in the opposite direction to maintain a tightseal until the tank pressure falls below the lower threshold.

Prior to the pressure seal or the vacuum seal breaking when the tankpressure reaches the respective upper or lower threshold, it isdesirable to have the seals urged as tightly as possible against theircorresponding seal surfaces to reduce the amount of leakage. That is, itis desirable to have the sealing force for the seals to be as high aspossible while still enabling the seals to break away when thecorresponding thresholds are reached. While increasing the strength orrating of the springs would increase the sealing force for theassociated seals, the increased spring forces may prevent the seals fromreleasing at the desired thresholds because greater pressure (or greatervacuum) within a tank would be need before the spring forces would beovercome.

Examples disclosed herein increase the sealing forces on the seals byincreasing the amount of force contributed by the pressure in the tankacting on the seals in a direction that urges the seals in engagementwith their respective sealing surfaces. In particular, the forceproduced by the tank pressure is increased by increasing the surfacearea on which the tank pressure acts to urge the seals in the directionof increased sealing engagement with their respective sealing surfaces.In some examples, the surface area is increased by including a diaphragmwithin a hatch that separates a chamber at a first pressurecorresponding to the pressure of the tank from a second chamber at asecond pressure corresponding to an ambient environment (e.g.,atmospheric pressure).

FIG. 1 is a schematic of a storage tank system 100 having a tank 102with an example thief hatch 104 mounted thereon. The storage tank system100 may be a component of a variety of applications. Furthermore, whilethe tank 102 is described as a storage tank, the tank 102 may be anysuitable fluid containing vessel used for any suitable purpose. In theillustrated examples, the tank 102 is coupled to an input device 106that may pump, discharge, or otherwise introduce a fluid into the tank102. Similarly, the tank 102 is coupled to an output device 108 toreceive or withdraw fluid from the tank 102. The input and outputdevices 106, 108 may be any type of fluid handling, storage, and/orprocessing components or equipment that are operatively coupled to thetank 102 substantially permanently or on a selective, temporary, and/orintermittent basis.

When a fluid is stored in the tank 102, pressure may build within thetank 102. For example, the input device 106 may discharge the fluid forstorage into the tank 102 at an elevated pressure. In some examples, itmay be desirable to vent or release pressure within an interior 110 ofthe tank 102 that is above a threshold pressure (e.g., a high pressurethreshold). As such, the tank 102 includes the thief hatch 104, which isconfigured to open (e.g., periodically open) fluid communication betweenthe interior 110 of the tank 102 and an ambient environment 112 externalto the tank 102. In particular, the thief hatch 104 may be configured toopen fluid communication between the interior 110 of the tank 102 andthe ambient environment 112 surrounding the tank 102 when the pressurewithin the interior 110 of the tank 102 exceeds an upper thresholdpressure. In this manner, vapor, fumes, and/or fluid may be vented fromthe tank 102, thereby decreasing the internal pressure of the tank 102.Once the internal pressure of the tank 102 falls below the upperthreshold pressure, the seal assembly of the thief hatch 104 mayautomatically close and re-seal, thereby blocking fluid communicationbetween the interior 110 of the tank 102 and the ambient environment112.

In some instances, pressure may decrease within the tank 102 to form avacuum (e.g., a negative pressure relative to the ambient environment112). For example, the output device 108 may pump fluid from the tank102. In some examples, it may be desirable to reduce an excessive vacuumwithin the tank 102 by allowing fluid (e.g., air) from the ambientenvironment 112 to be introduced into the interior 110 of the tank 102when the pressure is below a threshold pressure (e.g., a vacuum pressurethreshold). As such, the thief hatch 104 may be configured to open(e.g., periodically open) fluid communication between the interior 110of the tank 102 and an ambient environment 112 external to the tank 102in a similar manner to that described above for excess pressure in thetank 102. Thus, in some examples, the thief hatch 104 regulates thepressure within the tank 102 to be within upper and lower thresholds. Insome examples, the thief hatch 104 includes two different sealassemblies that cooperatively operate to selectively open or remainsealed at respective ones of the thresholds limiting the operating rangeof pressures maintained with the tank 102.

FIG. 2 is a cross-section of the example thief hatch 104 mounted on thetank 102 of FIG. 1. The thief hatch 104 includes a lid or cover 202 anda base 204. As shown in the illustrated example, the cover 202 ispivotably coupled to the base 204 via a hinge 206 formed using a firstpin 208 extending through both the cover 202, and the base 204. Thecover 202 includes a latch 210 that hooks onto a second pin 212 on thebase 204 to secure the cover 202 in a closed position. While a pin 212is shown in FIG. 2, the latch 210 may hook onto any other suitableelement attached to the base 204 (e.g., a ridge or protrusion integrallyformed in the base 204).

In the illustrated example, a pressure seal assembly 214 is removablycoupled to the cover 202. The example pressure seal assembly 214includes a center body 218 with a flange 220 on which is attached agasket or pressure seal 222. In some examples, the pressure seal 222 isa hollow seal with a generally c-shaped cross section to wrap around theflange 220. The example pressure seal assembly 214 also includes apressure spring 224 positioned to urge the center body 218 away from thecover 202. As shown in the illustrated example, when the cover 202 is inthe closed position, the pressure seal 222 abuts against a sealingsurface 226 on the base 204 of the hatch 104. Therefore, as the pressurespring 224 urges the center body 218 of the pressure seal assembly 214away from the cover 202, the pressure spring 224 urges the pressure seal222 against the sealing surface 226. In this manner, a relatively tightseal between the base 204 and the pressure seal 222 is maintained untilpressure within the tank 102 exceeds an upper threshold above an ambientpressure. When the tank pressure exceeds the upper threshold, thepressure seal 222 separates from the associated sealing surface 226 torelease pressure within the interior 110 of the tank 102 past the seal222 and out of the cover 202 to the ambient environment 112 exterior tothe tank 102.

In the illustrated example of FIG. 2, the thief hatch 104 includes avacuum seal assembly 228 that is operatively coupled to the pressureseal assembly 214. As shown in FIG. 2, the example vacuum seal assembly228 includes a stem 230 that extends through the center body 218 of thepressure seal assembly 214. In the illustrated example, the stem 230supports a gasket or vacuum seal 232 adjacent a first end 234 of thestem 230. The vacuum seal 232 is secured between a seal support 236 onone side of the vacuum seal 232 and a vacuum pallet disk 238 on theother side. In some examples, the vacuum seal 232 is a hollow seal.

In the illustrated example, a stop 240 is arranged to protrude from anouter diameter of the stem 230. As shown in the illustrated example, avacuum spring 242 is positioned between the stop 240 and the center body218 to urge the stem 230 upwards (from the perspective shown in FIG. 2)relative to the center body 218 of the pressure seal assembly 214. Theupward movement of the stem 230 is limited by the vacuum seal 232 cominginto contact with a sealing surface 244 on the center body 218. Thus,the vacuum spring 242 serves to urge the vacuum seal 232 against thesealing surface 244. In this manner, a relatively tight seal between thecenter body 218 and the vacuum seal 232 is maintained until pressurewithin the tank 102 drops below a lower threshold corresponding to avacuum (e.g., a negative pressure relative to an ambient pressure). Whenthe tank pressure is a vacuum below the lower threshold, the vacuum seal232 separates from the associated sealing surface 244 to allow pressureto be introduced into the interior 110 of the tank 102 from the ambientenvironment 112 after flowing under the cover 202, through the centerbody 218, and past the vacuum seal 232. In the illustrated example, theseal support 236, the vacuum seal 232, and the vacuum pallet disk 238are secured in place against a shoulder of the stem 230 via a nut.

As shown in FIG. 2, the cover 202 of the example thief hatch 104 definesa first chamber 246 that is enclosed by a diaphragm 248 coupled tosecond end 250 of the stem 230. The diaphragm 248 may be secured inplace between the cover 202 and an actuator case or housing 252 defininga second chamber 254 on the opposite side of the diaphragm 248 to thefirst chamber 246. In the illustrated example, the stem 230 includes abore 256 extending along a length of the stem 230 as a means for placingthe interior 110 of the tank 102 in fluid communication with the firstchamber 246. In this manner, the first chamber 246 is maintained at thepressure of the tank 102. In other examples, the interior 110 of thetank 102 may be placed in fluid communication with the first chamber 246via tubing, piping, or any other suitable conduit. In some examples, thethief hatch 104 includes a gauge port 258 to enable monitoring of thepressure within the tank 102. As shown in the illustrated example, thegauge port 258 may be on the cover 202 adjacent the first chamber 246that is maintained at the pressure of the tank 102.

Furthermore, in the illustrated example, the actuator housing 252includes a vent 260 to place the second chamber 254 in fluidcommunication with the ambient environment 112 exterior to the tank 102.As a result, the vent 260 provides a means for maintaining an area(e.g., the second chamber 254) adjacent the diaphragm 248 at thepressure of the ambient environment 112 (e.g., atmospheric pressure).

In the illustrated example, a pressure differential between the firstchamber 246 and the second chamber 254 produces a force urging thediaphragm 248 either (1) away from the tank 102 (when the tank pressurein the first chamber 246 is greater than the ambient pressure in thesecond chamber 254) or (2) towards the tank 102 (when the tank pressurein the first chamber 246 is less than the ambient pressure in the secondchamber 254). The amount of force generated on the diaphragm 248corresponds to the magnitude of the pressure differential multiplied bythe surface area of the diaphragm 248.

Inasmuch as the stem 230 is coupled to the diaphragm 248, the resultingforce acting on the diaphragm 248 at the second end of 250 of the stem230 is translated along the stem 230 to the stop 240 acting on thevacuum spring 242 as well as the vacuum seal 232 at the first end 234 ofthe stem 230. The force on the seal 232 transferred from the diaphragm248 is an additional force beyond that created by the pressuredifferential directly across the vacuum seal 232 (and associated sealsupport 236 and vacuum pallet disk 238). That is, in the illustratedexample, the tank pressure acts on the surface areas associated withboth the diaphragm 248 and the vacuum seal 232 to produce a larger forcethan is possible if the pressure were limited to acting only on thevacuum seal 232 directly. Thus, when the tank pressure is above theambient pressure of the ambient environment 112, the force of pressureacting on the diaphragm 248 combines with the force of pressure actingon the seal 232 directly (along with the spring force from the vacuumspring 242) to urge the vacuum seal 232 away from the tank 102 andagainst the associated sealing surface 244. In other words, thediaphragm 248, with the bore 256 in the stem 230 placing the firstchamber 246 in fluid communication with the tank 102, is a means forproviding a larger surface area on which the tank pressure is applied toincrease the sealing force of the vacuum seal 232 (when the tankpressure is greater than the ambient environment 112).

The additional sealing force urging the vacuum seal 232 against thesealing surface 244 on the center body 218 of the pressure seal assembly214 will urge the center body 218 away from the tank 102, therebyreducing the sealing force urging the pressure seal 222 against thecorresponding sealing surface 226 of the base 204. This may result inthe pressure seal 222 opening prematurely to release pressure before adesired upper threshold is reached. Accordingly, in some examples, theadditional force transferred to the pressure seal assembly 214 iscompensated for by a stronger pressure spring 224. In this manner, atighter seal for the vacuum seal 232 is achieved than is possible forother similar hatches without significantly impacting the threshold atwhich pressure may be released passed the pressure seal 222.Furthermore, the increased strength of the pressure spring 224 resultsin an increased sealing force between the pressure seal 222 and theassociated sealing surface 226 when the tank pressure is below thepressure of the ambient environment 112.

When the tank 102 is at a negative pressure (e.g., is in a vacuum state)relative to the ambient environment 112, the pressure differentialacross the diaphragm 248 produces an additional force urging the vacuumseal 232 towards the tank 102 and away from the associated sealingsurface 244. As with the pressure seal assembly 214, this additionalforce may cause the vacuum seal 232 to separate from the sealing surface244 prematurely to allow pressure into the tank before the lowerthreshold is reached. Accordingly, in some examples, the additionalforce transferred to the vacuum seal 232 is compensated for by astronger vacuum spring 242. In this manner, a tighter seal for thevacuum seal 232 is achieved at tank pressures above the lower thresholdwhile still enabling pressure to be introduced into the tank when thetank pressure drops below the lower threshold.

FIG. 3 is a cross-sectional view of another example thief hatch 300 thatmay be mounted to the tank 102 of FIG. 1. Similar to the hatch 104 shownin FIG. 2, the example hatch 300 shown in FIG. 3 includes a diaphragm302 secured between a cover 304 of the hatch 300 and an actuator housing306. The diaphragm 302 separates a first chamber 308 within the cover304 from a second chamber 310 within the actuator housing. In theillustrated example, the cover 304 includes a vent 312 to place thefirst chamber 308 in fluid communication with the ambient environment112 exterior to the tank 102. As a result, the vent 312 provides a meansfor maintaining an area (e.g., the first chamber 308) adjacent thediaphragm 302 at the pressure of the ambient environment 112 (e.g.,atmospheric pressure). Further, in the illustrated example, a flexibletube 314 or other suitable conduit extends from the actuator housing 306to a base 316 of the hatch 300 to place the interior 110 of the tank 102in fluid communication with the second chamber 310. In this manner, thesecond chamber 310 is maintained at the pressure of the tank 102.

A pressure differential across the diaphragm 302 results from adifference in pressure between the first chamber 308 (maintained at thepressure of the ambient environment 112) and the second chamber 310(maintained at the pressure of the tank 102). In the illustratedexample, a pressure differential arising from an over pressure withinthe tank 102 produces a force urging the diaphragm 302 towards the tank102. This is the opposite direction of movement to the diaphragm 248 ofFIG. 1 in over pressure conditions. However, as shown in the illustratedexample of FIG. 3, the movement of the diaphragm 302 is translated to astem 318 of a vacuum seal assembly 320 via a lever 322. The lever 322urges the stem 318 in the opposite direction to the diaphragm 302. As aresult, when the diaphragm 302 is urged towards the tank 102 (when thetank pressure is greater than the ambient pressure (e.g., an overpressure)), the stem 318 is urged away from the tank 102. The stem 318,in turn, urges a vacuum seal 324 of the vacuum seal assembly 320 awayfrom the tank 102, thereby contributing to the sealing force used toengage the vacuum seal 324 with the associated sealing surface 326.

Similar to the diaphragm 248 of FIG. 2, the amount of force generated onthe diaphragm 302 of FIG. 3 corresponds to the magnitude of the pressuredifferential between the first and second chambers 308, 310 multipliedby the surface area of the diaphragm 302. In FIG. 2, this force directlycorresponds to the force transferred to the vacuum seal 232. However,unlike FIG. 2, the force transferred to the vacuum seal 324 of FIG. 3from the diaphragm 302 is different than the pressure force acting onthe diaphragm 302 because of the lever 322. That is, based on thelocation of the rotation point or fulcrum 328 of the lever 322, theforce generated by the diaphragm 302 may be magnified to produce agreater sealing force at the vacuum seal 324. The fulcrum 328 may bepositioned at any suitable location along the lever 322 to define theresulting amount of force that contributes to the sealing force of thevacuum seal 324.

In the illustrated example of FIG. 3, the vacuum seal 324 is securedbetween a seal support 330 on one side of the vacuum seal 324 and avacuum pallet disk 332 on the other side. In some examples, the vacuumseal 324 is a hollow seal. In some examples, the vacuum seal 324 (andthe associated seal support 330 and vacuum pallet disk 332) is connectedto the stem 318 using a flexible or swivel joint (e.g., a ball joint) tofacilitate and/or improve the sealing engagement of the vacuum seal 232against the sealing surface 244 on the center body 218. In particular,as shown in the illustrated example, the vacuum seal assembly 320includes a ball attachment 334 that is includes a spherical surface 336that interfaces with the vacuum pallet disk 332 to enable the disk 332to swivel or change orientation. A sealing membrane 338 is positionedbetween the vacuum pallet disk 332 and the vacuum seal 324 anddimensioned to be secured between a shoulder on the ball attachment 334and the end of the stem 318 when the ball attachment 334 is attached(e.g., via threads) to the stem 318. Further, the sealing membrane 338is clamped between the vacuum pallet disk 332 and the vacuum seal 324via screws 340 extending therethrough.

While different example thief hatches 104, 300 have been shown anddescribed in FIGS. 2 and 3, many variations to the disclosed structuresmay be implemented in accordance with the teachings disclosed herein.For example, while a flexible joint for the vacuum seal assembly 320 isshown in FIG. 3, a similar joint could alternatively be implemented forthe vacuum seal assembly 228 of FIG. 2. A difference would be that theball attachment may include a bore that aligns with the bore 256 in thestem 230 of FIG. 2. Likewise, the fixed joint for the vacuum sealassembly 228 shown in FIG. 2 could be implemented in the thief hatch 300of FIG. 3. Further, the flexible tube 314 shown in FIG. 3 mayimplemented in the thief hatch 104 of FIG. 2 to place the interior 110of the tank 102 in fluid communication with the first chamber 246instead of relying on the bore 256.

From the foregoing, it will be appreciated that the above disclosedapparatus improve the sealing of pressure and vacuum seal assembliesinside a thief hatch while at pressures within upper and lower thresholdpressures while still enabling the relief of pressure or vacuum atdesired threshold pressures.

Although certain example methods, apparatus and articles of manufacturehave been disclosed herein, the scope of coverage of this patent is notlimited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe claims of this patent.

What is claimed is:
 1. An apparatus, comprising: a base attachable to atank; a cover attached to the base via a hinge; and a vacuum sealassembly couplable to the cover; a diaphragm coupled to a stem of thevacuum seal assembly, the diaphragm moveable in response to a pressuredifferential between a first pressure within the tank and a secondpressure exterior to the tank; and an actuator housing coupled to thecover, the diaphragm secured between the cover and the actuator housingto separate a first chamber within the actuator housing from a secondchamber within the cover.
 2. The apparatus of claim 1, wherein thevacuum seal assembly is to urge a vacuum seal into sealing engagementwith a sealing surface when the first pressure is above a vacuumpressure threshold.
 3. The apparatus of claim 2, further including apressure seal assembly couplable to the cover, the pressure sealassembly to urge a pressure seal against the base, the pressure sealassembly including the sealing surface against which the vacuum seal isurged, the vacuum seal assembly to urge the vacuum seal against thepressure seal assembly via a spring force.
 4. The apparatus of claim 3,wherein the diaphragm produces an additional force that combines withthe spring force to urge the vacuum seal against the pressure sealassembly when the first pressure is greater than the second pressure. 5.The apparatus of claim 4, further including a pressure spring to urgethe pressure seal against the base, the pressure spring to overcome thecombination of the spring force and the additional force.
 6. Theapparatus of claim 1, wherein the actuator housing includes a vent toplace the first chamber in fluid communication with an exteriorenvironment.
 7. The apparatus of claim 1, wherein the cover includes avent to place the second chamber in fluid communication with an exteriorenvironment.
 8. The apparatus of claim 1, wherein the first pressurewithin the tank is provided to the second chamber from the tank via abore extending along a length of the stem.
 9. The apparatus of claim 1,wherein the first pressure within the tank is provided to the firstchamber from the tank via a tube extending between the base and theactuator housing.
 10. The apparatus of claim 1, wherein the diaphragm iscoupled to the stem via a lever.
 11. The apparatus of claim 1, whereinthe vacuum seal is coupled to the stem via a flexible joint.
 12. Anapparatus, comprising: a base attachable to a tank; a cover pivotablycoupled to the base via a hinge; an actuator housing coupled to thecover; and a diaphragm disposed between the actuator housing and thecover, the diaphragm separating a first chamber within the actuatorhousing from a second chamber within the cover, a pressure differentialbetween a first pressure in the first chamber and a second pressure inthe second chamber to produce a force urging a vacuum seal against asealing surface.
 13. The apparatus of claim 12, wherein the force fromthe pressure differential combines with a spring force from a vacuumspring to urge the vacuum seal against the sealing surface.
 14. Theapparatus of claim 12, further including a vent on the actuator housingto maintain the first pressure at an ambient pressure surrounding thetank.
 15. The apparatus of claim 12, wherein the second pressurecorresponds to a tank pressure within the tank.
 16. The apparatus ofclaim 12, further including a stem, the diaphragm coupled to the stemproximate a first end of the stem, the vacuum seal coupled to the stemproximate a second end of the stem opposite the first end.
 17. Theapparatus of claim 16, wherein the stem includes a bore extending alonga length of the stem to place the second chamber in fluid communicationwith the tank.
 18. An apparatus, comprising: a base of a thief hatchattachable to a tank; a cover of the thief hatch attached to the basevia a hinge; a vacuum seal disposed within the thief hatch, the vacuumseal to be urged against a sealing surface with a sealing force thatincludes a pressure force generated from a first pressure in the tankwhen the first pressure is above a second pressure in an ambientenvironment exterior to the tank, the vacuum seal to let pressure intothe tank from the ambient environment when a tank pressure is less thana vacuum pressure threshold; and means for providing a larger surfacearea on which the first pressure is applied to increase the sealingforce.
 19. The apparatus of claim 18, further including: a stem, thevacuum seal coupled to the stem proximate a first end of the stem,wherein the means for providing the larger surface area includes adiaphragm coupled to the stem proximate a second end of the stem, afirst side of the diaphragm enclosing a chamber within the cover of thethief hatch; and means for placing the chamber in fluid communicationwith an interior of the tank.
 20. The apparatus of claim 19, furtherincluding means for maintaining an area adjacent a second side of thediaphragm at the second pressure.